1. Field of the Invention
The present invention is directed to an x-ray diagnostics system of the type wherein a transparency signal of the examination subject is used as a basis for setting x-ray exposure parameters.
2. Description of the Prior Art
U.S. Pat. No. 4,455,669 (the teachings of which are incorporated herein by reference) discloses automatic transparency matching, particularly for mammography. This is a technique for controlling the radiation dose by shutting off the x-ray source after a time which is determined in part on a transparency signal of the examination subject, which is representative to the thickness and density of the subject. In this known system the selection of the exposure parameters (kV, filter) ensues manually.
The beam quality (dependent on anode material, filtering, tube voltage) is critical for the image quality obtainable in mammography. The beam quality most frequently employed at present (Mo anode, Mo filter, tube voltage 25 kV through 30 kV) supplies high-contrast images given subjects transparent to radiation. Given thick or dense subjects, the beam exposure is high; moreover, dense gland tissue is only inadequately portrayed. Employing a tube having W-Mo anode dish whose emitted radiation is prefiltered with various cut-off filters (for example, Mo, Ru, Rh) offers one possibility of matching the beam quality to the subject thickness density.